Many biochemical processes involve the coupled transfer of electrons and protons. This is a key step, for instance, for a range of enzymes (e.g., cytochrome c oxidase, photosystems I and II, cytochromes P450) and in the trapping of reactive oxygen species (e.g., by vitamin E and superoxide dismutases). The goal of the proposed research is to develop a fundamental and predictive understanding of these processes. This understanding will be valuable in a range of biochemical systems, much as the current knowledge of pure electron transfer has been very valuable. The proposed work encompasses a variety of compounds, reactions, and techniques to uncover the essential features of the chemistry. Hydrogen atom transfer reactions are a primary focus of the proposal, building on the recent discovery that a range of H-atom transfer reactions follow the Marcus cross relation. The Marcus approach enables prediction of reaction rates and provides a new fundamental intuition for these reactions, based on driving force and intrinsic barriers. The intrinsic barriers can be measured through studies of self-exchange rates, which will be determined for a number of compounds. The relationship between the intrinsic barriers for electron, proton, and hydrogen atom transfer will be examined. Extensions to hydride transfer reactions are discussed, including possible application of the Marcus approach. New chemical systems will be developed in which an intramolecular proton transfer is coupled to intermolecular electron transfer. Such proton-coupled electron transfer (PCET) processes are very common, as in the oxidation of the tyrosine Z-histidine unit in photosystem II. It will be determined whether proton transfer precedes, succeeds, or is concerted with electron transfer in such systems. The reasons for adopting one mechanism or another will be probed, using the intrinsic barriers and thermodynamics of the reactions. Chemical reactions that involve metal peroxide complexes will also be examined, both reactions of isolated peroxides and reactions that could form O-O bonds. The proposed work takes a broad view - studying iron, cobalt, manganese, ruthenium and osmium systems and a variety of types of reactions - in order to provide new and valuable insights into the various kinds of proton-coupled electron transfer that occur in biology.